(1) Field of the Invention
The present invention relates to a PCM coder and decoder circuit having a digital balancing network. More particularly, the present invention pertains to a PCM coder and decoder circuit, such as that provided in a subscriber line interface circuit of a telephone switching system, which has a coder adapted to convert an analogue signal applied through a two-wire to four-wire converter into a digital signal, a decoder adapted to decode the digital signal into an analogue signal and apply this analogue signal to the two-wire to four-wire converter, and a balancing network adapted to remove any echo signal which is produced by a portion of the output of the decoder which undesirably enters the coder through the two wire-four wire converter.
(2) Description of the Prior Art
To suppress the echo signal produced through the two-wire to four-wire converter, it is conventional practice to employ a circuit having characteristics collectively approximated to the amplitude and phase-frequency characteristics of all the devices included in the echo path, such as an A/D converter, a D/A converter, etc., in addition to the two -wire to four-wire converter, as shown in, for example, The Bell System Technical Journal, Vol. 50, pp.785 to 813 (March 1971). Accordingy, the circuit configuration is complicated, and the size of the circuit is increased, which means that it is difficult to realize excellent echo suppression characteristics economically. Moreover, it is not easy to change the adequate characteristics when the external conditions from the PCM encoder and decoder, and two-wire load impedances will be changed.